1. Field of the Invention
The present invention relates to a driving-force transmission device and an image forming apparatus that employs the driving-force transmission device.
2. Description of the Related Art
A driving-force transmission device that is employed in an image forming apparatus transmits a rotary driving force from a driving source such as a motor to a rotating unit (drive target) such as an image carrier. When the rotating unit is detachable from the image forming apparatus, the driving-force transmission device typically includes a gear (driving-force input unit) that receives the rotary driving force from the driving source, a rotary shaft mounted on the gear, and a coupling member that is mounted on the rotary shaft and couples to a coupled portion of the rotating unit. A driving-force transmission device having such a configuration is disclosed in, for example, Japanese Patent Application Laid-open No. 2002-328499. With such a configuration, fluctuation in rotational velocity of the rotating unit largely depends upon adverse effect by a gear and a coupling member. The adverse effect by the gear includes eccentricity of the gear and eccentric error in mounting the gear on a rotary shaft, and the adverse effect by the coupling member includes eccentricity of the coupling member, eccentric error in mounting the coupling member on rotary shaft, and an engaging gap between the coupling member and a coupled portion.
The adverse effect by the eccentricity of the gear and the coupling member can be suppressed by improving molding accuracy.
Furthermore, the adverse effect by the gap between the coupling member and the coupled portion can be suppressed by applying spline engagement in which the coupling member and the coupled portion can be molded with low shape error and can be easily detached. In the spline engagement, one of the rotary shaft of the rotating unit and a boss portion of a driving-force transmitting member is formed into a spline shaft, and a spline hole is formed in the other. The spline shaft is inserted into the spline hole to mesh external teeth on the spline shaft with internal teeth in the spline hole.
Moreover, the adverse effect by the eccentric error in mounting the gear or the coupling member on the rotary shaft can be suppressed by mounting the gear and the coupling member on the rotary shaft without causing backlash.
Recently, resin has been increasingly used for forming a gear and a coupling member on the beneficial aspects of vibration, noise, and cost. On the other hand, metal is often used for a rotary shaft on the beneficial aspect of torsional stiffness. However, with a combination use of parts molded from different materials, linear coefficient of expansion differs between the parts. Therefore, a gap may be formed at engaging portion between the rotary shaft and the gear or between the rotary shaft and the coupling member due to temperature change in an operating environment and heat generated from a driving source in an image forming apparatus. This leads to backlash of the gear or the coupling member relative to the rotary shaft and eccentric rotation of the gear or the coupling member, resulting in fluctuation in rotational velocity of the rotating unit.
To address such fluctuation, the inventors of the present invention have invented a driving-force transmission device that employs a driving-force transmitting member including a rotary shaft unit, a gear, and a coupling member, all of which are integrally formed using the same material of resin. The use of such a driving-force transmission device does not cause the backlash even if the driving-force transmitting member thermally expands. Therefore, the eccentric rotation of the gear and the coupling member can be sufficiently suppressed.
However, the inventors found that the above driving-force transmitting member causes the following problem.
The driving-force transmitting member needs to be supported in a rotatable manner in an image forming apparatus to allow transmission of a rotary driving force that is input to the gear, from the coupling member to a drive target. Therefore, the driving-force transmitting member needs to be supported at least at two support portions by a support member such as a side plate on a side of an image forming apparatus via a metal sleeve bearing. With the use of such a sleeve bearing, sliding friction between an outer circumferential surface of the rotary shaft unit of the driving-force transmitting member and an inner circumferential surface of the sleeve bearing can be suppressed low over a prolonged period. In this manner, generally, the driving-force transmitting member can be rotatably supported by the support member for a long period. However, resin that forms the driving-force transmitting member has the linear coefficient of expansion larger than metal that forms the sleeve bearing. Accordingly, if the driving-force transmitting member thermally expands by temperature rise in an operating environment or heat from a heat source such as a motor, a gap between the outer circumferential surface of the rotary shaft unit and the inner circumferential surface of the sleeve bearing is reduced, so that friction loading between the outer circumferential surface of the rotary shaft unit and the inner circumferential surface of the sleeve bearing increases, i.e., rotation load on the driving-force transmitting member increases, leading to overload on the motor to be stopped.
The countermeasure for the above is to reduce a diameter of the rotary shaft unit at the support portion to as small as possible to suppress a dimensional change in the rotary shaft unit when the driving-force transmitting member thermally expands. However, in the driving-force transmitting member employing an engagement in which one end of the rotary shaft unit engages with an engaging target arranged concentrically with the rotary shaft unit (for example, spline engagement) as a configuration of the gear and the coupling member, a diameter of the one end of the rotary shaft unit needs to be increased to assure the strength of the one end. Therefore, when the driving-force transmitting member thermally expands, the dimensional change of the rotary shaft unit is increased at the support portion in the one end (large-diameter portion), resulting in overload on the motor to be stopped.
Still worse, at the support portion on the side of the large-diameter portion, frictional heat between the outer circumferential surface of the rotary shaft unit and the inner circumferential surface of the sleeve bearing causes the rotary shaft unit to be melted and adhered to the sleeve shaft. Under such a circumstance, if rotation of the sleeve bearing is restricted relative to the support member (including the case where the sleeve bearing cannot rotate relative to the support member due to increased frictional force caused by the thermally-expanded sleeve bearing), the driving-force transmitting member can not be rotated, causing overload on the motor to be stopped.
Such a problem occurs not only in the case where the driving-force transmitting member is formed of resin and the sleeve bearing is formed of metal, but also in the case where the driving-force transmitting member is formed from a material having linear coefficient of expansion larger than that for the sleeve bearing.
Furthermore, this can occur also between the sleeve bearing and the support member. If the sleeve bearing is formed from a material having linear coefficient of expansion larger than that for the support member, frictional force between the sleeve bearing and the support member increases by the thermally-expanded sleeve bearing, so that a motor may stop due to overloading. Moreover, if the rotation of-the sleeve bearing is restricted relative to the driving-force transmitting member (including the case where the driving-force transmitting member cannot rotate relative to the sleeve bearing due to the thermal expansion of the driving-force transmitting member), the diving-force-transmitting unit can not be rotated and the motor may stop due to overloading.